1. Field of the Invention
This invention relates to an array antenna, and more particularly to an electronically 360 degree steerable passive array antenna capable of steering the radiation beams and nulls of a radio signal.
2. Description of Related Art
An antenna is used wherever there is wireless communication. The antenna is the last device through which a radio signal leaves a transceiver and the first device to receive a radio signal at a transceiver. Most antennas are designed to radiate energy into a “sector” which can be regarded as a “waste” of power since most of the energy is radiated in directions other than towards the intended transceiver. In addition, other transceivers experience the energy radiated in other directions as interference. As, such a great detail of effort has been made to design an antenna that can maximize the radiated energy towards the intended transceiver and minimize the radiation of energy elsewhere.
A scanning beam antenna is one type of antenna known in the art that can change its beam direction, usually for the purpose of maintaining a radio link between a tower and a mobile terminal. Early scanning beam antennas were mechanically controlled. The mechanical control of scanning beam antennas have a number of disadvantages including a limited beam scanning speed as well as a limited lifetime, reliability and maintainability of the mechanical components such as motors and gears. Thus, electronically controlled scanning beam antennas were developed and are becoming more important in the industry as the need for higher speed data, voice and video communications increases in wireless communication systems.
Referring to FIG. 1, there is illustrated a traditional electronically controlled scanning beam antenna 100 known in the art as a phased array antenna 100. The phased array antenna 100 has an RF signal input 102 connected to a network of power dividers 104. The power dividers 104 are connected to a series of phase shifters 106 (eight shown). The phase shifters 106 are used to control the phase of a radio signal delivered to an array of radiating elements 108 (eight shown). The phased array antenna 100 produces a radiation beam 110 that can be scanned in the direction indicated by arrow 112. As can be seen, the phased array antenna 100 has a complex configuration and as such is costly to manufacture. These drawbacks become even more apparent when the number of radiating elements 108 become larger.
Referring to FIG. 2, there is illustrated another traditional electronically controlled scanning beam antenna 200 that was described in U.S. Pat. No. 6,407,719 the contents of which are hereby incorporated by reference herein. The array antenna 200 includes a radiating element 202 capable of transmitting and receiving radio signals and one or more parasitic elements 204 that are incapable of transmitting or receiving radio signals. Each parasitic element 204 (six shown) is located on a circumference of a predetermined circle around the radiating element 202. Each parasitic element 204 is connected to a variable-reactance element 206 (six shown). A controller 208 changes the directivity of the array antenna 200 by changing the reactance Xn of each of the variable-reactance elements 206. In the preferred embodiment, the variable-reactance element 206 is a varactor diode and the controller 208 changes the backward bias voltage Vb applied to the varactor diode 206 in order to change the capacitance of the varactor diode 206 and thus change the directivity of the array antenna 200. This array antenna 200 which incorporates varactor diodes 206 has several drawbacks when it operates as a high frequency transmit antenna. These drawbacks include low RF power handling, high linearity distortion and high loss of the RF energy. Accordingly, there is a need to address the aforementioned shortcomings and other shortcomings associated with the traditional electronically controlled scanning beam antennas. These needs and other needs are satisfied by the electronically steerable passive array antenna and method of the present invention.